One effective way to improve the thermal efficiency of a gas turbine is to boost the temperature at the gas inlet of the turbine. It is also desirable to suppress increased emission of NO.sub.x from the combustor, which supplies combustion gases to the turbine, and to improve the heat resistance of the turbine and its cooling capacity.
Since the combustor is exposed to temperatures of 1500 to 2000.degree. C., it must be properly cooled so that the temperature of its exterior wall panels remains in the allowable range as it experiences thermal stress.
Generally, combustors in gas turbines are cooled by running the air to be used for combustion along their inner wall panels, and by forcing air inside these wall panels in order to cool the metal components so that their temperature is lower than that of the combustion gases.
However, if air is used to cool the turbine, the air used for cooling and the air that leaks from the cooling channels is released into the main gas flow. This air makes it more difficult to improve the capacity of the gas turbine and decrease the emission of NO.sub.x.
This has led to proposals for using steam instead of air as the cooling medium.
In the past few years, combined power plants have received a great deal of publicity. These power plants make use of both gas and steam turbines in order to increase their generating efficiency (i.e., their thermal efficiency). A schematic diagram of a combined power plant is shown in FIG. 6. The gas turbine generating system comprises generator 40, compressor 41, combustor 42 and gas turbine 43. A steam turbine generating system, which comprises boiler 45, steam turbine 46, on whose output shaft 46a generator 40 is mounted, and steam condenser 47, is installed on the gas turbine. The exhaust gases from the gas turbine 43 are fed into boiler 45. The boiler water supplied from steam condenser 47 is heated and vaporized, and this steam is used as the drive source for steam turbine 46.
In this sort of combined power plant, there is an abundant supply of steam, which can easily be tapped, and steam has a higher thermal capacity to transmit heat than air does. Recently, engineers have been studying the use of steam instead of air as a cooling medium for the parts of the turbine that experience high temperatures. However, if the steam, which has been used to cool the hot portions of the turbine in a combined power plant, is released into the main gas flow, the temperature of the flow will drop, and the thermal efficiency of the turbine will decrease. For this reason it has been suggested that the steam used for cooling should be entirely recovered and used as drive steam for the steam turbine.
FIG. 6 illustrates how this method of steam cooling would work. As indicated by the dotted lines in the drawing, the steam generated in waste heat recovery boiler 45 is extracted and conducted to the hot portions of the combustor or other areas of the turbine which need to be cooled. All the steam used for cooling is then recovered and used as drive steam for steam turbine 46. This method enables a gas turbine 43 to be realized with a temperature at its gas inlet port in excess of 1500.degree. C., and it also improves the overall efficiency of the combined power plant.
Although the use of steam instead of air as the cooling medium in the combustor of a gas turbine has been given a great deal of consideration, it is still at the conceptual level and has not yet been put into practice.
One reason for this is because it can be difficult to create steam-cooling channels in a combustor wall, which has complex forms, especially by a conventional laser or electrospark machining.
For steam cooling, it is necessary to use high pressure steam as a cooling medium. This demands a strong enough structure for forming the steam channels, but in fact, there is no actual structure for such a steam-cooling to fulfil this demand in the market.
There is yet another reason why there is no such steam-cooled structure in the actual market. In the steam-cooled combustor, there must be a steam supplying means and a steam recovering means around the combustor. In addition to this requirement, it is also important not to allow leakage of the steam from the steam system. It is, however, not easy to fulfil all of these requirements because of structural reasons. This made it difficult to make such a steam-cooled combustor in the actual market.
It is naturally not practical to use the same structure and the same concept used for an air-cooled combustor as a steam-cooled combustor, because it does not fulfil the requirements for steam-cooled combustor.